BELLEEK, Northern Ireland, (Reuters) - British Prime Minister Theresa May arrived on Thursday in Northern Ireland, whose border with EU-member Ireland has become one of the biggest impediments to reaching a deal to leave the European Union.

May is still reeling from the resignation last week of former Foreign Secretary Boris Johnson, who in parliament on Wednesday singled out her treatment of the border as the biggest mistake of her negotiations with the EU for a smooth exit from the bloc next year.

May's Brexit 'white paper', the policy document which prompted Johnson's resignation, proposes negotiating the closest possible commercial links for goods trade with the bloc to protect businesses and to fulfill a commitment to avoid having infrastructure on the border.

The 500-kilometre (300 mile) border has been largely invisible since army checkpoints were taken down after a 1998 peace deal ended three decades of violence between the region's pro-British majority and an Irish nationalist minority. Over 3,600 died.

Politicians have warned that the re-imposition of physical infrastructure when it becomes the EU's external border would anger Irish nationalists in Northern Ireland who aspire to unification with the Republic of Ireland and help militants opposed to the peace deal to recruit new members.

May has refused to accept a "backstop" solution proposed by the European Union in which Northern Ireland would remain closely aligned with the European Union's single market and customs union on the grounds that it would create a border between Northern Ireland and the rest of the United Kingdom.

But she has said she hopes that close regulatory alignment on goods trade between the United Kingdom and the European Union would allow the border to remain open.

Johnson on Wednesday told parliament that May had unnecessarily let the "readily soluble" border issue "become so politically charged as to dominate the debate" pushing May towards a close alignment with the EU he described as a "miserable, permanent limbo".

The first stop in May's visit was the border village of Belleek in Fermanagh, the home county of pro-British Democratic Unionist Party leader Arlene Foster.

She arrived in Belleek by helicopter to a "smattering of applause", a Sky News reporter said, before touring a pottery factory with Foster.

"This visit will enable Mrs May to speak with people who live, work and travel across the much talked about Irish border on a daily basis," Foster said in an earlier statement.

May will also meet members of Foster's party, which provides 10 votes in Britain's lower house of parliament that the prime minister needs to govern.

On Friday she will meet the leadership of the largest Irish nationalist party, Sinn Fein.

Northern Ireland has been without a regional government since January 2017, when Sinn Fein pulled out of a power-sharing government with the DUP and extended talks to re-establish it have failed.